The present application claims priority under 35 U.S.C. xc2xa7 119 of Austrian Patent Application No. 1565/2001, filed on Oct. 3, 2001, the disclosure of which is expressly incorporated by reference herein in its entirety.
1. Field of the Invention
The invention relates to a hot-working steel article, in particular, a tool for forming metals and alloys at elevated temperatures. Components, above all tools, that are stressed at elevated temperatures, e.g., extrusion die matrices, forging tools, die casting dies, extrusion dies, mandrels and the like, require materials which have mechanical properties commensurate with the intended stress at temperatures of, if necessary, 550xc2x0 C. and higher and which retain these properties during an extended operating time.
2. Discussion of Background Information
By working according to the prior art, however, a high hardness and toughness of the material, a low plastic deformation under extreme stresses, high wear resistance, retention of hardness and good fatigue strength properties of a hot-working steel in the temperature range of above 550xc2x0 C. cannot be achieved simultaneously to the desired extent by alloying technology methods. At given thermal and mechanical strains, the chemical composition and thermal treatment of an article (e.g., tool) should thus be selected such that the profile of the material properties achievable thereby comes as close as possible to meeting all requirements, although a shorter service life of the article often has to be accepted in this case.
For a long time materials science has been faced with the problem of improving the long-term use properties at elevated temperature of articles made of hot-working steel and of providing an alloy with which a high toughness of the material in combination with a high degree of hardness thereof can be achieved by a thermal treatment, so that the risk of fracture, even with shock like strain of a piece, and the plastic deformation and wear are minimized thereby. In that regard, the retention of hardness and the heat conductivity of the material also are to be taken into consideration.
One skilled in the art is aware that retention of hardness and unchanged maintenance of the mechanical properties at elevated temperature of a thermally treated steel article are accomplished by special carbides that can develop at carbon concentrations in the range of 0.5% by weight and with chromium contents of 3 to 5% by weight Cr of the alloy, with molybdenum-tungsten and vanadium contents further increasing the heat resistance thereof. The common hot-working steels essentially have contents, in % by weight, of 0.35 to 0.665 C, 2.0 to 7.0 Cr, 1.5 to 8.0 Mo and/or 1.5 to 18.0 W and 0.4 to 2.0 V, where vanadium can be replaced by higher molybdenum or, in particular, by higher tungsten concentrations.
In order to achieve a high hardenability of the hot-working steel with good retention of hardness and wear resistance even when thermally treating articles of large diameter, EP 0249855 suggests to use a steel composition of essentially, in % by weight, C=0.42 to 0.5, Mn=0.35 to 0.6, Si=0.8 to 1.2, Cr=5.8 to 6.2, Mo=1.85 to 1.95, V=0.7 to 0.9. An improvement in toughness, hardness, strength and wear resistance is achieved with the above alloy composition compared to a steel according to AISI type H 13. However, tempering to a hardness of more than 58 HRC causes a coarse grain formation of the microstructure and disadvantageous losses in toughness.
In order to improve the mechanical high-temperature properties, in particular under cyclical strain, the use of a cobalt-containing hot-working steel produced by powder metallurgy (U.S. Pat. No. 6,015,446) has also been proposed.
It is known from AT 403 058 to use a hot-working steel with increased aluminum contents for tools for the non-cutting heat-shaping of metals and alloys. This steel, although quite suitable for elevated working temperatures, exhibits an embrittlement tendency at hardness values above 58 HRC.
An alloy consisting essentially, in % by weight, of C=0.3 to 0.5, Si less than 0.9, Mn less than 1.0, Cr=2.0 to 4.0, Mo=3.5 to 7.0, 0.3 to 1.5 V and/or Ti and/or Nb, Al=0.005 to 0.1 is proposed in EP 0632139 as a material for hot-working tools that must have a heat conductivity of more than 35 W/m K, in order to thereby achieve a lower stress on the tool surface and a flatter temperature gradient within the tool to avoid thermoshock- and tension cracks.
EP 0939140 discloses a hot-working steel consisting essentially of, in % by weight, C=0.25 to 0.79, Cr=1.10 to 7.95, Mo=0.56 to 3.49, V=0.26 to 1.48, Fe=balance. In the above alloy the contaminants and accompanying elements are restricted to improve the properties of the formed material at high temperatures. High values for the high-temperature strength, the high-temperature toughness and the high-temperature wear resistance are achieved with this measure after a thermal treatment of the material to a hardness of lower than HRC=56, but a strong scattering of the values of the respective mechanical properties was detected at high temperature after thermal treatment to a hardness of the article of higher than 58 HRC.
A powder metallurgically produced hot-working steel that is characterized by a content of 1.5 to 2.5% by volume of carbides of the MC type is known from WO 00/26427. At hardness values above 58 to 59 HRC, at which tools are increasingly to be provided for cold forming, higher as well as lower MC contents than 2.5 to 1.5% by volume have a detrimental effect on the flexural impact strength.
The disclosures of all documents mentioned above are hereby expressly incorporated by reference herein in their entireties.
It would be desirable to eliminate the shortcomings in the above prior art and to provide a hot-working steel article that with high degrees of material hardness and similar strength properties simultaneously ensures toughness values at a considerably higher level and, in combination with good heat conductivity, provides an improved wear resistance at elevated temperatures and an effective prolongation of the service life of the part under intensified and, optionally, shock like stresses.
The present invention provides a hot-working steel article comprising a material which comprises, in % by weight, 0.451 to 0.598 carbon, 0.11 to 0.29 silicon, 0.11 to 0.39 manganese, 4.21 to 4.98 chromium, 2.81 to 3.29 molybdenum, 0.41 to 0.69 vanadium, with the balance being iron, contaminants and accompanying elements. (Unless stated otherwise, the weight percentages given herein are based on the total composition.)
In one aspect, the ratio C/V of the material is 0.82 to 1.38. In another aspect, the ratio (Cr+Mo+V)/C is 15.2 to 18.4. In yet another aspect, the material comprises less than 0.1% by weight of W. In a still further aspect of the article, the content of carbides which are formed upon solidification of a melt on which the material is based is less than 0.45 vol.-%.
Furthermore, the material may comprise not more than 0.005% by weight of sulfur and/or not more than 0.007% by weight of phosphorus and/or not more than a total of 0.010% by weight of (sulfur+phosphorus) and/or not more than 0.15% by weight of nickel and/or not more than 0.1% by weight of cobalt and/or not more than 0.1% by weight of copper and/or not more than a total of 0.25% by weight of (nickel+cobalt+copper) and/or not more than 0.02% by weight of aluminum and/or not more than 0.001% by weight of magnesium and/or not more than 0.001% by weight of calcium and/or not more than a total of 0.02% by weight of (aluminum+magnesium+calcium). As used herein and unless stated otherwise, the phrase xe2x80x9cnot more thanxe2x80x9d in combination with weight percentages includes 0% by weight, i.e., absence of the respective component. Also, it should be understood that the values of the weight percentages given herein are approximate values, i.e., not limited to the exact values stated.
In another aspect of the article, the material comprises less than 0.025% by weight of nitrogen. In still another aspect, the material comprises not more than 0.005% by weight of arsenic and/or not more than 0.003% by weight of bismuth and/or not more than 0.005% by weight of tin and/or not more than 0.002% by weight of zinc and/or not more than 0.002% by weight of antimony and/or not more than 0.002% by weight of boron and/or not more than a total of 0.009% by weight of (arsenic+bismuth+tin+zinc+antimony+boron).
Furthermore, the material may have a hardness of at least 58 HRC, an impact strength of at least 170 J and a notched impact strength (Charpy U) in longitudinal direction of at least 11 J. For example, the material may have a hardness of at least 59 HRC at room temperature, and at a temperature of 500xc2x0 C. it may have an impact strength of at least 180 J and a notched impact strength (Charpy U) in longitudinal direction of at least 14 J. (The impact strength is determined according to xe2x80x9cStahl.Eisen-Prxc3xcfblxc3xa4tterxe2x80x9d (SEP) 1314 (Steel Test Specification), the determination of the notched impact strength is to be carried out according to DIN EN 10045. Both documents are hereby expressly incorporated herein by reference in their entireties).
The material may have been heat-treated at a temperature below 1080xc2x0 C., for example, at a temperature in the range of 1040xc2x0 to 1060xc2x0 C.
The article may be a tool, in particular, a tool for forming metals and alloys at elevated temperatures. For example, it may be selected from extrusion die matrices, forging tools, die casting dies, extrusion dies, and mandrels.
The present invention also provides an alloy for a hot-working steel article. The alloy comprises, in % by weight, 0.451 to 0.598 carbon, 0.11 to 0.29 silicon, 0.11 to 0.39 manganese, 4.21 to 4.98 chromium, 2.81 to 3.29 molybdenum, 0.41 to 0.69 vanadium, with the balance being iron, contaminants and accompanying elements. Further aspects of this alloy are those indicated above with respect to the material of the article of the present invention.
Furthermore, the present invention provides a process for making a hot-working steel article. According to said process, an article comprising a material which comprises, in % by weight, 0.451 to 0.598 carbon, 0.11 to 0.29 silicon, 0.11 to 0.39 manganese, 4.21 to 4.98 chromium, 2.81 to 3.29 molybdenum, 0.41 to 0.69 vanadium, with the balance being iron, contaminants and accompanying elements, is heat-treated at a temperature below 1080xc2x0 C. to a hardness of the material of at least 58 HRC, an impact strength of at least 170 J and a notched impact strength (Charpy U) in longitudinal direction of at least 11 J.
In one aspect of this process, the temperature for the heat treatment is in the range of 1040xc2x0 to 1060xc2x0 C. In a further aspect, the article is heat-treated to a hardness of the material of at least 59 HRC at room temperature, and an impact strength of at least 180 J and/or a notched impact strength (Charpy U) in longitudinal direction of at least 14 J, both at a temperature of 500xc2x0 C. Further aspects of this process are those indicated above with respect to the material of the article of the present invention.
The advantages achieved with the invention essentially are that a solid solution hardening with a low proportion of carbides is made possible through the alloying technique or through a respective balanced concentration of carbon and the carbide-forming elements in the steel, respectively. A hardening to values above 58 HRC can be performed at lower austenization temperatures, e.g., of 1080xc2x0 C. or lower, corresponding to the dissolution of carbon proceeding more readily, which promotes the fine-grain quality of the material and is advantageous with respect to a high toughness of the material. In other words, it has been found that certain concentrations within selected limits of carbon and of the elements forming special carbides and monocarbides promote a desired solid solution hardenability and suppress a carbide hardening or a hardness-increasing separation of coarser carbides at the expense of the matrix hardness, respectively, during thermal treatment.
According to the invention it is important, due to the interactions of the elements, or better, the activities of the reacting elements, to match them with one another. A carbon content of at least 0.451% by weight ensures the minimum activity of carbon for distorting the lattice of the matrix crystals and a carbide-forming tendency at the provided chromium, molybdenum and vanadium concentrations. Carbon contents of the alloy of higher than 0.598% by weight, although promoting the wear resistance, may have a disadvantageous effect on the hardness and toughness of the article. The chromium content should synergistically be set between 4.21 and 4.98% by weight. Cr concentrations that are higher than 4.98% by weight may shift the retention of hardness of the hot-working steel towards lower temperatures, while chromium values lower than 4.21% by weight may cause a reduced tendency to form special carbides. The activity of molybdenum and vanadium with respect to carbon, which is determined by the contents thereof, is of significance in view of the matrix hardening during thermal treatment. It has been found that Mo has a kind of masking effect on V and, at contents of at least 2.81% by weight, retards VC monocarbide separation and, thus a matrix depletion. On the other hand, with molybdenum contents higher than 3.29% by weight, the affinity towards carbon is so high that a dissolution of the same during austenization of the article may be significantly retarded or reduced. A minimum content of 0.41% by weight of V is desirable for a corresponding development of the secondary hardness during tempering of the hot-working steel article; contents of V higher than 0.69% by weight may increase the tendency to form monocarbides, which, as has been found, can also have a negative impact with respect to a reduction of the heat conductivity of the steel. Silicon in concentrations between 0.11 and 0.29% by weight is desirable for an efficient deoxidation of the liquid steel. Si contents higher than 0.29% by weight may impair the material toughness at the intended use temperatures. Manganese is provided for binding the sulfur. When using modern desulfurization methods, it may be possible to keep the manganese contents as low as 0.11% by weight. Manganese concentrations higher than 0.39% by weight may impair the high-temperature toughness of the steel, in particular, in combination with further grain boundary active elements.
It can be seen from the above statements that a synergistic selection of the respective concentrations of carbon, silicon, manganese, chromium, molybdenum and vanadium according to the invention provides a hot-working steel article with a high degree of hardness of 58 HRC and higher and, at the same time, a superior toughness through thermal treatment.
It is advantageous for the contents of carbon and vanadium to be selected such that the ratio: concentration of V divided by that of C equals 0.82 to 1.38. Through this ratio within the stated range, monocarbide formation is suppressed, in terms of formation-kinetics, in favor of the matrix content, and the solid solution hardenability is favored.
An increase in hardness in combination with an increased retention of hardness, an improved high-temperature wear resistance and service life of a hot-working steel article can be achieved if the ratio of the concentrations of chromium+molybdenum+vanadium divided by the carbon content is between 15.2 and 18.4.
Very surprisingly, because molybdenum and tungsten are considered interchangeable as far as their tendency to form carbides is concerned, it was found that tungsten promotes the tendency to form primary carbides and, in particular, favors segregations and possibly, grain growth, where a reduction in segregations by annealing of the hot-working steel is considerably reduced by tungsten. According to the invention, the tungsten content of the hot-working steel article desirably is selected to be less than 0.1% by weight.
The hot-working steel article preferably has a proportion of carbides formed in the melt during the solidification thereof of less than 0.45% by volume. On the one hand, a depletion of the solid solutions with respect to carbon seems to be prevented and a further increase in hardness seems to be attainable thereby and, on the other hand, as was found, an increase in the heat conductivity of the hot-working steel article may be achieved. An improvement in the heat conductivity by means of a reduction of the carbide proportion in the material has not yet been scientifically ascertained, but might be due to interface kinetics and/or the properties of the carbides.
In a further embodiment of the invention, a reduction of the contaminants and/or accompanying elements which is advantageous for improved use properties of the hot-working steel article at elevated temperatures is to be provided. The correspondingly provided individual and collective concentrations of the elements are recited above and in the appended claims, respectively.
It has proved advantageous to set the upper limit for the nitrogen content of the alloy at 0.025% by weight, because nitrogen forms stable nitrides with the carbide-forming elements Cr, Mo and V, which nitrides may cause disadvantages with respect to the thermal treatment.
A hot-working steel article with particularly high performance profile can be produced if the value of at least one of the following mechanical properties of the steel at a temperature of 500xc2x0 C. is equal to or higher than:
impact strength: 180 J
notched impact strength in longitudinal direction: Charpy-U 14 J
and the hardness thereof at RT is at least 59 HRC.
The decisive advantages of a fine-grained microstructure with regard to a high material toughness with concomitant high hardness values can be obtained with the alloy according to the invention if the hardening temperature for the thermal treatment for setting the mechanical properties is lower than 1080xc2x0 C., in particular 1050xc2x0 C., plus/minus 10xc2x0 C.
Other exemplary embodiments and advantages of the present invention may be ascertained by reviewing the present disclosure and the accompanying drawing.